The present invention relates to an optical switch, in particular, relates to such a switch for switching sequentially a plurality of optical paths in an optical communication system.
Generally, in a communication system, when the reliability of a system is not sufficient, a redundant standby component is provided for the component which has the lowest level of reliability, and when the normal component becomes unavailable, that component is switched to the standby component. In particular, in an optical communication system utilizing an optical fiber cable, lasers and semiconductor photo-detectors, the reliability of those elements is unknown, and then, the standby components for those components are installed. In switching the components, it is important that the switching does not affect the transmission performance, and that the switching element itself has a high enough level of reliability.
Now, the switching operation of a semiconductor laser will be analyzed below. When the degradation of the normal semiconductor laser is recognized, the standby semiconductor laser is switched ON. It should be noted that a semiconductor lasers does not provide the perfect optical waveform initially, although the optical waveform is completed in a short time. When the output optical waveform is not complete, that output waveform must not be coupled with the communication line. When the standby laser provides the complete output optical waveform, the output of the laser is switched from the normal laser to the standby laser, and then, the normal laser becomes a standby laser, and the standby laser becomes a normal laser. Then, when the switching between the two lasers is finished, the first normal laser which is now the standby laser is switched OFF. In the above switching operation, an optical switch must provide the isolation between a normal laser and a standby laser in order to prevent the undesirable interference between the two lasers. Of course, the insertion loss of an optical switch must be as small as possible.
A prior optical switch is shown in FIGS. 1A, 1B and 1C, in which a pair of optical fibers confront each other, and the optical fiber 3 at the reception side displaces the position, and then, the switching operation is performed. That is to say, as shown in FIGS. 1A and 1B, the reception side optical fiber 3 confronts with either the optical fiber 1 or the optical fiber 2, and the switching operation between the optical fibers 1 and 2 is performed. However, it should be appreciated as shown in FIG. 1C that there is a transient position that the reception optical fiber 3 locates at the intermediate position between the input optical fibers 1 and 2. In that transient position, the reception optical fiber 3 might not couple with the optical fibers 1 and 2, or even if the reception optical fiber 3 couples with the optical fibers 1 and 2, the combined optical energy transmitted to the reception optical fiber 3 from the optical fibers 1 and/or 2 might be reduced during the switching transient duration.
FIGS. 2A through 2C show another prior optical switch, in which a switching operation is accomplished through the displacement of a prism. In FIG. 2A, the input beam 9 at the input port 5 is reflected twice by the prism 8 which is installed in the housing 4, and then, said input beam 9 appears at the output port 7 as the output beam 11. On the other hand, another beam 10 at another input port 6 does not appear at the output port 7. On the other hand, when the prism 8 displaces to the position shown in FIG. 2B, the input beam 10 at the input port 6 is directly coupled with the output port 7, but the input beam 9 at the input port 5 does not appear at the output port 7. Therefore, the displacement of the prism 8 provides the switching operation of the optical beams.
In analyzing the switching operation of the optical switch of FIGS. 2A and 2B, it should be noted that there is a transient position of the prism 8 that both the input beams 9 and 10 pass the prism 8. That is to say, as shown in FIG. 2C, when the input beam passes the vertical angle 8-a of the prism 8, the other input beam 10 might pass the other vertical angle 8-b of the prism 8. In that occasion, due to the incompleteness of the vertical angles 8-a and 8-b of the prism 8, the combined beam 11 by the input beams 9 and 10 is attenuated a little. Therefore, the output optical beam 11 is attenuated in a short time during the switching transient duration.
As described above, a prior optical switch has the disadvantage that the level of the output beam is attenuated in the switching transient duration. In that transient duration, the S/N (signal to noise ratio) is lowered at the succeeding optical repeater, and thus, the communication quality is deteriorated. It should be noted that said low level in the transient duration can not be compensated by the operation of an AGC (automatic gain control), because the response time of an AGC system is slower than the transient time of an optical switch.
FIG. 3 shows the curves the bit error rate at the succeeding optical repeater due to the reduction of the optical level in the switching transient. In FIG. 3, the vertical axis shows the bit error rate, and the horizontal axis shows the amount of the decrease of the optical level in the switching transient in dB. In FIG. 3, the mark ratio of the transmission pulse train is 1/2, the transmission speed is 280 Mb/second, the waveform is rectangular with the duty ratio 50%, the ON/OFF ratio of a laser is 10 dB, the wavelength is 1.3 .mu.n, the receiving photodetector is GeAPD, utilizing the FCRO equalizer, and the thermal noise is 6 pA/.sqroot.Hz. The S/N margin in FIG. 3 shows the margin of the S/N allowed for the succeeding optical repeater which has not switched a laser yet, and said S/N margin shows the amount to the reference S/N (in the embodiment, the reference S/N is 22.55 dB which corresponds to the error rate 10.sup.-11). In FIG. 3 it should be noted that when the decrease of the error rate due to the optical switching must be less than 10.sup.-7, and when the S/N margin is 3 dB, the decrease of the optical power during the transient duration of the switching must be less than 1.6 dB. However, a prior optical switch can not satisfy that condition.